The two-dimensional body of Figure 3-6 has the initial surface and internal temperatures as calculated. At time zero the 500ºC face is suddenly lowered to 30ºC. Taking ∆x = ∆y = 15 cm and α = 1.29 × 10−5 m2/s, calculate the temperatures at nodes 1, 2, 3, and 4 after 30 min. Perform the calculation using both a forward- and a backward-difference method. For the backward-difference method use only two time increments. Take k = 45 W/m · ºC.